The first method to becoming an admin, is to head on over to your Players tab and click "Add Admin" next to your username. Reboot your server, and you are now an Admin.
If the first option does not work, you can try manually adding your SteamID64 to the adminlist in SFTP. Or by going to the Console tab and running the command admin add steamID64 0 then rebooting your server to become an admin.

If none of those options are working, log into your server. Then go to the Console tab on the server page, enter into the console the command listplayerids from there look for your ingame username and the 3-4 digit ID. Using the ID, then run the command admin add id 0 save and restart your server and you are now an admin!